Autologous mouse testicular cells derived from the seminiferous tubules activate suppressor T cells which inhibit mixed cell reactions in vitro. Generation of cytotoxic T cells in vitro is reduced in the presence of syngeneic germ cells. Mice were repeatedly injected intrarectally with xenogeneic semen (swine), and the cytotoxic T lymphocyte potential of the mice was followed with time. Depressed immune responses were seen early in the course of injection (first 5 weeks), but responses returned to normal levels as the mice continued to be stimulated. No evidence for Kaposi's-like lesions were detected in the skin of these mice.